Increasing restrictions on the amount and types of volatiles which may be released in work environments have prompted the development of radiation curable coating compositions which are free of volatile solvents that must be evaporated during the curing of the composition. These solvent-free coating compositions are known as 100 percent reactive systems; that is, each component of the composition reacts and becomes incorporated into the cured coating when the composition is exposed to radiation. Particularly useful as reactive components in radiation curable coating compositions are monofunctional or polyfunctional acrylate or methacrylate esters.
In many instances, it is desired to apply an acrylate or methacrylate containing radiation curable composition as a clear protective coating to a porous substrate such as, for example, paper or paperboard. A problem has been observed in such applications in that the uncured coating composition has a tendency to penetrate into the porous substrate. Consequently, a portion of the coating composition which has penetrated into the substrate remains substantially uncured and causes a permanent greying, or wet staining, of the coated substrate. To date no completely satisfactory method has been devised for inhibiting penetration into a porous substrate by an acrylate or methacrylate containing radiation curable composition for a sufficient period of time to allow the composition to be cured to a solid state.